Advances in electrochemical storage and conversion devices have significantly expanded the capabilities of these systems in a variety of fields including portable electronics, aerospace technologies, communications and biomedical instrumentation. State of the art electrochemical storage and conversion devices are specifically engineered to have designs and performance attributes supporting specific target application requirements and operating environments. Such advanced electrochemical storage systems include high energy density batteries exhibiting low self discharge rates and high discharge reliability for implanted medical devices; inexpensive light weight rechargeable batteries for portable electronics, and high capacity batteries capable of providing high discharge rates over short time intervals for military and aerospace applications.
Widespread implementation of this diverse suite of advanced electrochemical storage and conversion systems continues to motivate research directed to expanding the functionality of these systems to enable the next generation of high performance device applications. Growth in the demand for high power portable electronic products, for example, has created enormous interest in developing safe, light weight primary and secondary batteries with higher energy densities. The demand for miniaturization in the field of consumer electronics and instrumentation also continues to stimulate research into novel design strategies for reducing the sizes, masses and form factors of high performance batteries. Further, developments in the fields of electric vehicles and aerospace engineering has also created a significant need for highly reliable batteries exhibiting high energy densities and high power densities for a range of useful operating environments.
Many advances in electrochemical storage and conversion technology are directly attributable to discovery and integration of new materials for battery components. Lithium battery technology, for example, continues to rapidly develop, at least in part, due to the discovery of novel electrode and electrolyte materials for these systems. From the pioneering identification of intercalation host materials for positive and negative electrodes to the development of high performance non-aqueous electrolytes, the discovery and optimization of novel materials for lithium battery systems have revolutionized their design and performance capabilities. As a result of these advances, lithium based battery technology is currently preferred for certain commercially significant applications including primary and secondary electrochemical cells for portable electronic systems.
Advances in materials strategies and cell designs for lithium battery technology have realized primary and secondary electrochemical cells capable of providing useful device performance including: (i) large energy densities (e.g., ≈150 Wh kg−1), (ii) high cell voltages (e.g. up to about 3.6 V), (iii) substantially constant (e.g., flat) discharge profiles, (iv) long shelf-life (e.g., up to 10 years), (v) good cycling characteristics, and (vi) compatibility with a range of operating temperatures (e.g., −20 to 60 degrees Celsius). As a result of these beneficial characteristics, primary and secondary lithium batteries are widely employed as power sources for many portable electronic devices, such as cellular telephones and portable computers, and for other important device applications in the fields of biomedical engineering, sensing, military communications, and lighting.
Primary lithium battery systems typically utilize a lithium metal negative electrode for generating lithium ions. During discharge, lithium ions are transported from the negative electrode through a liquid phase or solid phase electrolyte and undergo intercalation reaction at a positive electrode comprising an intercalation host material. Dual intercalation lithium ion secondary batteries have also been developed, wherein lithium metal is replaced with a second lithium ion intercalation host material providing the negative electrode. In lithium ion secondary cells, simultaneous lithium ion insertion and de-insertion reactions allow lithium ions to migrate between the positive and negative intercalation electrodes during discharge and charging cycles. Incorporation of a lithium ion intercalation host material for the negative electrode has the significant advantage of avoiding the use of metallic lithium which is susceptible to safety problems upon recharging attributable to the highly reactive nature and non-epitaxial deposition properties of lithium. Useful intercalation host materials for electrodes in lithium cells include carbonaceous materials (e.g., graphite, cokes, subfluorinated carbons etc.), metal oxides, metal sulfides, metal nitrides, metal selenides and metal phosphides. U.S. Pat. Nos. 6,852,446, 6,306,540, 6,489,055, and “Lithium Batteries Science and Technology” edited by Gholam-Abbas Nazri and Gianfranceo Pistoia, Kluer Academic Publishers, 2004, are directed to lithium and lithium ion battery systems which are hereby incorporated by reference in their entireties.
Electrolytes for lithium electrochemical cells are limited to nonaqueous materials given the extremely reactive nature of lithium with water. Several classes of nonaqueous electrolytes have been successfully implemented for lithium electrochemical cells including: (i) solutions of lithium salts dissolved in organic or inorganic solvents, (ii) ionically conducting polymers, (iii) ionic liquids and (iv) fused lithium salts. Nonaqueous electrolyte solutions comprising lithium salts dissolved in polar organic solvents are currently the most widely adopted electrolytes for primary and secondary lithium cells. Useful solvents for these electrolytes include polar solvents that facilitate dissociation of lithium salts into their ionic components. Polar solvents exhibiting useful properties for lithium cell electrolytes include linear and cyclic esters (e.g., methyl formate, ethylene carbonate, dimethyl carbonate and propylene carbonate), linear and cyclic ethers (e.g., dimethoxiethane, and dioxolane) acetonitrile, and γ-butyrolactone. Lithium salts in these electrolyte systems are typically salts comprising lithium and complex anions that have relatively low lattice energies so as to facilitate their dissociation in polar organic solvents. Lithium salts that have been successfully incorporated in electrolytes for these systems include LiClO4, LiBF4, LiAsF6, LiSbF6, LiAlCl4 and LiPF6 provided at concentrations ranging from 0.01 M to 1M.
Successful implementation of polar organic solvent based electrolyte systems for primary or secondary lithium batteries involves a number of considerations involving their chemical and physical properties. First, the electrolyte must be capable of forming a stable passivation layer on the surfaces of the electrode that does not result in a significant voltage delay at the onset of discharge and is capable of rapid reformation upon high current discharge. Second, the electrolyte must be chemically stable with respect to electrolytic degradation for relevant electrode material and discharge conditions. Third, the electrolyte must exhibit a useful ionic conductivity. State of the art electrolytes for these systems, for example, exhibit ionic conductivities at 25 degrees Celsius greater than or equal to about 0.005 S cm−1. Other physical properties of electrolytes useful for providing enhanced performance in electrochemical cells include thermal stability, low viscosity, low melting point, and high boiling point.
The power output of many state of the art lithium cells is currently limited by the conductivity of electrolytes which determines, in part, the internal resistance of these systems. Accordingly, substantial research is currently directed toward developing electrolytes for primary and secondary lithium cells providing large ionic conductivities for accessing higher device performance. A number of strategies have been developed for increasing the ionic conductivities of polar organic solvent based electrolyte systems for primary or secondary lithium batteries. Many of these strategies involve providing additives to the electrolyte to enhance dissolution of a lithium salt while at the same time maintaining chemical and electrochemical stability under discharge and charging conditions.
Anion receptors are a class of compounds that have been recently developed as additives to increase the ionic conductivity of nonaqueous electrolyte solutions (See, e.g., U.S. Pat. Nos. 6,022,643, 6,120,941, and 6,352,798). Anion receptors enhance the ionic disassociation of lithium salts in low dielectric solvents by incorporating non-hydrogen bonded electrophilic groups that participate in complex formation reactions with anions of the lithium salt provided to the electrolyte. Some anion receptor additives have been demonstrated to enhance the dissolution of specific lithium salts in a manner resulting in an increase in solubility by several orders of magnitude. Anion receptor additives encompass a wide range of compounds including fluorinated boron-based anion receptors, such as boranes, boronates and borates having electron withdrawing ligands, polyammonium compounds, guanidiniums, calixarene compounds, and aza-ether compounds. Successful integration of anion receptors in lithium batteries, however, depends on a number of key factors. First, the anion receptor must be stable with respect to electrolyte decomposition under useful discharge and charging conditions. Second, anion receptors should be capable of releasing (or de-complexing) complexed anions so as not to hinder intercalation reactions at the electrodes. Third, the anion receptor itself preferably should not participate in intercalation with the intercalation host material, and if it does participate in such intercalation reactions it should not result in mechanically induced degradation of the electrodes.
Additives have also been developed to impart other useful chemical and physical characteristics to polar organic solvent based electrolytes for lithium cells. U.S. Pat. No. 6,306,540 (Hiroi et al.), for example, provides additives for improving the stability of nonaqueous electrolytes by minimizing gas formation decomposition reactions involving lithium salts and their dissociation products. This reference discloses electrolyte compositions having a LiF additive provided to a solution of LiPF6 in a nonaqueous organic solvent. At least partial dissolution of the LiF additive generates fluoride ions in the nonaqueous electrolyte which is reported to suppress gas forming decomposition reactions involving PF6− anions. The reference notes, however, that very little fluoride ion is generated in the electrolyte due to the inherently low solubility of LiF in the nonaqueous organic solvents evaluated. The reference reports, for example, that due to the poor solubility of LiF in the electrolytic solution it was difficult to dissolve 0.2% by weight of LiF (˜0.077 M) at room temperature.
As will be clear from the foregoing, there exists a need in the art for nonaqueous electrolytes exhibiting chemical and physical properties useful for electrochemical conversion and storage systems. Nonaqueous electrolytes are needed that exhibit large ionic conductivities and good stability for use in primary and secondary lithium electrochemical cells. Specifically, a need exists for additives for enhancing the solubility and stability of lithium salts in nonaqueous electrolytes for primary and secondary lithium electrochemical cells.
Further, there exists generally a need in the art for methods providing for enhanced solubility and/or dissolution of fluorides, including inorganic fluorides that typically exhibit very low solubilities in many solvent environments. Processes and compositions providing enhanced solubility of fluorides are needed to allow new chemistries to take place in the solution phase, including aqueous and nonaqueous phases. A broad range of potential applications exists for methods and compositions for enhancing the solubility of fluorides including surface fluorination, and organic and inorganic fluorination using soft chemistry methods. One example of a class of such reactions involves surface fluorination for the purpose of enhancing corrosion resistance. Sources of solution phase fluoride ions are particularly needed that do involve the use of, or formation of, highly corrosive HF in solution and/or gas phases.
Table 1 provides a summary of solubility data for a range of inorganic fluorides in water. As shown in Table 1, many solid state inorganic fluorides (MFn), for example CdF2, CoF2, FeF3, MnF2, NaF, NiF2, ZnF2, ZrF4, AlF3, BaF2, CaF2, CuF2, FeF2, InF3, LiF, MgF2, PbF2, SrF2, UF4, VF3-3H2O, BiF3, CeF3, CrF2/CrF3, GaF3, LaF3, NdF3, and ThF4, are poorly soluble in water and many organic solvents. Other fluorides, such as CsF, RbF, KF, SbF3 and AgF, readily dissolve into water at the ambient temperatures. When hydrolysis is not a problem, insoluble element fluorides can be prepared as water precipitates by halide metathesis or by the reaction of aqueous hydrofluoric acid with the appropriate element oxide, hydroxide, carbonate or with the element itself. As discussed above, however, the use of hydrofluoric acid has significant drawbacks given its highly corrosive and toxic nature.
Accordingly, the dissolution of insoluble fluorides is currently a great challenge in chemical science and technology. Among other advantages, it can provide fluorine rich solutions for new chemical synthesis through solution reactions or for appropriate physical properties of dissolved fluorinated species. Specifically, methods and compositions providing enhanced solubility of fluorides may provide an important tool for accessing solution phase fluoride compositions useful for solution phase and surface phase synthetic pathways. As discussed above, methods and compositions providing enhanced solubility of fluorides would also enable new electrolyte solutions for many applications, including electrosynthesis, electrodeposition, and electropassivation, and in electrochemical energy storage and conversion systems such as primary and secondary batteries, electrochemical double-layer capacitors and fuel cells.
TABLE 1Summary Of Solubility Data For A Range Of Inorganic FluoridesSolubilitySolubility in Waterin WaterFluoride solubilityFluoride(g/100 ml)(mol/L)VVS (Very VeryAgF17213.5Soluble)VVSCsF57337.7VVSHFN/AN/AVVSRbF30028.8VVSSbF349227.5VVSTlF24511.0VS (Very Soluble)BeF2N/AN/AVSKF10217.6VSNH4F83.522.6VSSnF2N/AN/AVSTaF5N/AN/AVSVF4N/AN/ALS (Low Soluble)CdF24.360.29LSCoF21.40.14LSFeF35.920.52LSMnF21.020.11LSNaF4.130.98LSNiF22.560.26LSZnF21.550.15LSZrF41.50.09VLS (Very LowAlF30.50.0595Soluble)VLSBaF20.1610.0092VLSCaF20.00160.0002VLSCuF20.0750.0074VLSFeF2N/AN/AVLSInF30.040.0023VLSLiF0.1340.0515VLSMgF20.0130.0021VLSPbF20.0670.0027VLSSrF20.0210.0017VLSUF40.010.0003VLSVF3—3H2ON/AN/AIS (Insoluble)BiF3ISCeF3ISCrF2/CrF3ISGaF3ISLaF3ISNdF3ISThF4d (Decomposed)BF3dB4F, BrF3, BrF5dCoF3dGeF2/GeF4dHg2F2/HgF2dNbF5dOsF6dPF3/PF5dRhF3dSF4/SF6dSnF4dTeF4dUF6dVF5dWF6